Prospective challengers to silicon, the long-reigning king of semiconductors for computer chips and other electronic devices, have to overcome silicon’s superb collection of materials properties as well as sophisticated fabrication technologies refined by six decades of effort by materials scientists and engineers. Graphene, one of the latest contenders, has a rather impressive list of features of its own but has lacked a key characteristic of all semiconductors, an energy gap (band gap) in its electronic band structure. A multi-institutional collaboration under the leadership of researchers with Berkeley Lab and the University of California, Berkeley, have now demonstrated that growing an epitaxial film of graphene on a silicon carbide substrate results in a significant band gap, 0.26 electron volts (eV), an important step toward making graphene useful as a semiconductor.

Left: In graphene’s electronic band structure, the conduction (orange) and valence (purple) bands just meet at the Fermi energy (EF), so there is no band gap. Right: When a graphene layer is grown on a silicon carbide substrate), broken symmetry opens a gap (D) between the valence and conduction bands around the so-called Dirac energy (ED), as shown in the ARPES intensity map (lower right), but below the Fermi energy.

When a Gap is Good

Like an "unrolled" carbon nanotube, graphene is a densely packed single layer of carbon atoms arranged in a hexagonal pattern like a honeycomb. Although derived from graphite, the form of carbon in pencil leads, these two-dimensional graphene sheets may hold the promise of a new generation of faster, smaller, cheaper, and more durable computer chips than today's silicon-based devices. For example, even at room temperature, electrons can fly through the sheet while undergoing none of the collisions with atoms that generate heat and limit the speed and size of silicon-based devices. Also, because carbon has the highest melting point of any element, electronic devices made from graphene may be able to operate at much higher temperatures than possible today.

However, before graphene can be engineered into transistors or other electronic devices, a feature called an energy gap (because these energies are forbidden to electrons) must be introduced into graphene’s electronic band structure. While there are several promising efforts underway to induce such a gap, such as addition of impurities (doping) or the fabrication of geometrically confined structures with one or more dimensions measured in nanometers, Zhou et al. have demonstrated that growing graphene epitaxially on a silicon carbide substrate so that the atoms on each side of the interface maintain their registry could be a more reliable approach for generating a band gap.

First produced as a free-standing layer in 2004, graphene's characteristics—ballistic electron transport (i.e., without electron scattering), electrical conductivity controllable by chemically doping or by an electric field, high thermal conductivity, and high quality and strength—quickly stamped it as a possible material for future generations of semiconductor devices that are faster, smaller, cheaper, and more durable than today’s silicon-based devices. However, as a two-dimensional sheet of carbon atoms arranged in a hexagonal pattern, graphene lacks a gap between the top of its valence band and bottom of its conduction band because the two carbon atoms in its crystallographic unit cell see the same atomic environments, a symmetry that causes the two bands to just touch at the vertices of the Brillouin zone (a kind of unit cell in reciprocal or momentum space).

With increasing numbers of graphene layers on the substrate, the size of the energy gap decreased with thickness and all but disappeared at four layers as the graphene becomes more like bulk graphite.

Several promising efforts are underway to induce such a gap by breaking the symmetry (see ALSNews, Vol. 275), but the Berkeley-led group has taken a new approach. In brief, the group grew epitaxial layers of graphene on silicon carbide substrates by thermal decomposition of a silicon carbide surface oriented so that the silicon atoms were exposed. The interaction between the remaining carbon atoms and the underlying substrate resulted in a graphene layer configured in such a way that one of the carbon atoms in each unit cell has a neighboring atom in the atomic layer below and one does not, thus breaking the symmetry.

Working at ALS Beamlines 12.0.1 and 7.0.1 (the Electronic Structure Factory), the group members used angle-resolved photoemission to investigate the electronic structure of the epitaxial graphene. Measurements of the photoemission intensity as functions of the photoelectron kinetic energy and the photoelectron momentum (derived from the angle of emission) yielded a map of the electron band structure (energy vs. momentum) with a sizable energy gap of 0.26 eV at the Brillouin zone vertices. However, the Fermi energy (maximum energy occupied by electrons) was well up into the conduction band, whereas for a normal semiconductor the Fermi energy would be in the band gap.

In additional experiments with increasing numbers of graphene layers, the team found that the size of the energy gap decreased with thickness and all but disappeared at four layers. Finally, detailed measurements of the change in photoemission intensity symmetry around the vertices from six-fold to three-fold near the energy where the extrapolated valence and conduction bands would just meet (known as the Dirac point, see ALSNews Vol. 277) were consistent with the proposed symmetry breaking mechanism, provided that the expected buffer layer of carbon atoms was present between the graphene and the silicon carbide.

Next on the agenda are finding ways to control the width of the band gap, perhaps by using a different substrate material with a different graphene–substrate interaction strength, and to move the Fermi energy from the conduction band into the band gap to allow transistor action.

Research Funding: National Science Foundation; U.S. Department of Energy, Office of Basic Energy Sciences (BES); and Berkeley Lab Laboratory Directed Research and Development. Operation of the ALS is supported by BES.